1. Field of the invention
This invention relates to an improvement in gas-liquid phase separation in an ebullated bed process. Specifically, the invention relates to the use of a hydroclone to separate gas from hot, high pressure recycle liquid thereby reducing recycle pump cavitation, preventing ebullated bed collapse.
2. Description of Other Relevant Methods in the Field
The ebullated bed process comprises passing concurrently flowing streams of liquids or slurries of liquids and solids and gas through a vertically cylindrical vessel which contains catalyst. The mass of catalyst is placed in random motion in the liquid medium, and has a gross volume dispersed through the liquid medium greater than the volume of the mass when stationary. This technology has found commercial application in the upgrading of heavy liquid hydrocarbons or converting coal to synthetic oils.
The process is generally described in U.S. Pat. No. Re 25,770 to Johanson incorporated herein by reference. A mixture of hydrocarbon liquid and hydrogen is passed upwardly through a bed of catalyst particles at a rate such that the particles are forced into random motion as the liquid and gas pass upwardly through the bed. The catalyst bed motion is controlled by a recycle liquid flow so that the catalyst particles do not rise above a definable level in the reactor. Vapors along with the liquid which is being hydrogenated pass through that upper level of catalyst particles into a substantially catalyst free zone and are removed at the upper portion of the reactor.
In an ebullated bed process substantial amounts of hydrogen gas and light hydrocarbon vapors are present which rise through the reaction zone into the catalyst free zone. Liquid is both recycled to the bottom of the reactor and removed from the reactor as product from this catalyst free zone. Vapor is separated from the liquid recycle stream before being passed to the recycle pump suction. The recycle pump (ebullating pump) maintains the expansion (ebullation) and random motion of catalyst particles at a constant and stable level. Gases or vapors present in the recycled liquid materially decrease the capacity of the recycle pump as well as alter the flow pattern within the reactor, thus decreasing stability of the ebullated bed.
Reactors employed in a catalytic hydrogenation process with an ebullated bed of catalyst particles are designed with a central vertical conduit which serves as the downcomer for recycling clear liquid from the catalyst free zone above the ebullated catalyst bed to the suction of a recycle pump to recirculate the liquid through the catalytic reaction zone. The recycling of liquid from the upper portion of the reactor serves to ebullate the catalyst bed thereby maintaining uniform temperature through the reactor and stabilizing the catalyst bed.
U.S. Pat. No. 4,221,653 to Chervenak et al. describes a separator apparatus for separating vapor from liquid in an ebullated bed process. The apparatus comprises a frusto-conical cup in which are inserted a plurality of conduits. The conduits are positioned so that higher conduits carry gas-rich fluid and lower conduits carry liquid-rich fluid. The separator apparatus of the Chervenak et al. patent and those like it with a plurality of conduits are referred to as a tubular cup. The generic term for the recycle gas-liquid separator apparatus in an ebullating bed process is recycle cup.
It is a critical feature of the tubular cup that the upflowing liquid-gas mixture rising from the reaction zone passes through the fluid intake conduits of the separation apparatus and that all conduits are below the reactor liquid level. After passage through the separation apparatus, the gas portion rises to the space above the phase separation zone. Part of the liquid portion is returned through a downcomer conduit and recycled to the reaction zone, while the remaining liquid portion is withdrawn from the reactor as liquid product. The returned part of the liquid portion passes through the downcomer to a recirculating pump, then passes through a liquid-gas distributor means, together with fresh liquid and gas feed to maintain uniform upward fluid flow through the ebullated catalyst bed. The liquid and vapor effluent may be withdrawn separately from the upper portion of the reactor. In this case a second interface between liquid and vapor is established. Vapor is drawn from above the interface. If desired, liquid and vapor portions may be withdrawn together through a single conduit extending into the reactor to a point above the separator apparatus in the essentially solids free zone.
U.S. Pat. No. 3,677,716 to Weber et al. discloses an enclosed gas-liquid disengaging apparatus used in an ebullated bed process. The patent is most significant for its teaching that cyclones were tested and found to withdraw catalyst from the ebullated bed.
The design of cyclone separators such as those particularly adapted to the separation of vapors or solids from liquids, known as hydrocyclones or hydroclones is well known in the art. Examples of these design methods include API Manual of Refinery Wastes, Vol. Atmospheric Emissions, Chapter 11, May 1975 (API Pub. 931); Hydroclones: Dimensions and Performance by Zanker, Chemical Engineering, May 9, 1977 and U.S. Pat. No. 4,544,486.
There is a need for an effective means of separating gas from recycled liquid in an ebullated bed process, thereby increasing steady state operating time.